How to Calculate Your Zone Lengths If You Can't Measure Them

... (due for instance to finished basement ceilings or indeterminable travel inside walls) and get respectably within the Ballpark. It's easy, and all you need to measure are two temperatures. Useful for calculating the zones friction head loss, or for calculating the cost to redo them, if for no other purpose.

Step 1: Monitor the temperature drop of your boiler that results from the opening of the zone for which you wish to measure the length, and its dump of cold water into the boiler. Do this only when the zone is fully cold and do it over several firings to get a decent average drop. For each measurement record the boilers initial temperature, and its temperature low after the zone dumps all of its cold water into the boiler.

Step 2: Look up the gallons of water capacity for your boiler.

Step 3: Build your equation.

(Boiler Gal. x Boiler Temp Initial) + (Loop Gal. x Loop Temp Initial) = (Loop plus Boiler gallons x Boiler Low Temp)

Example:
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Boiler initial temp = 170 degrees
Boiler temp after zone dump = 155 degrees
Cold zone water temp = room temperature = 70 degrees
Boilers capacity in gallons = 25 gal.

1) (25 gal. x 170 deg.) + (Unknown_Loop Gal. x 70 deg.) = ([Unknown_Loop Gal. + 25 Boiler Gal.] x 155 deg.)

2) 4,250 + 70*Unknown_Loop_Gallon_degrees = 155*Unknown_Loop_Gallon_Degrees + 3,875

3) 375 = 85*Unknown_Loop_Gallon_Degrees

4) Calculated Gallons in Loop = 375/85 = 4.412 gal.

5) Determine how many gallons per foot your zone loops pipe contains. For 3/4" Type M this is 0.0268/ft.

6) 4.412/0.0268 = 164.63 feet

Final answer is: This 3/4" Type M Copper zone is about 165 feet in length

Proof test equation:

(25 gal x 170 deg.) + (4.412 Gal. x 70 deg.) = (29.412 Total Gal. x Unknown_Boiler_Low_Degrees)
4,250 + 308.84 = 29.412 x Unknown_Boiler_Low_Degrees
4558.84 = 29.412 x Unknown_Boiler_Low_Degrees
4558.84/29.412 = 155 degrees

155 degrees for the boilers low temp in the proof equation matches 155 degrees as measured, so we can conclude by this proof that this loop is around 165 feet in length, and the means whereby we achieved this figure are valid.

I should mention that generally hot water baseboards and cast iron radiators are against cold outside walls, and T-Stats are on warmer inside walls, and there is likely to be about a 3 to perhaps 4 degree difference between them, so if your loops T-Stat is set at 70 degrees, then the water entering your boiler from this cold zone is more likely to be 66 or 67 degrees than it is to be 70 degrees.

Using 66 degrees for the cold zone water temperature in the above example I get 157 feet for the zones length, instead of 165 feet. This small change will likely give you a bit better precision, although the calculated difference between 66 degree and 70 degree zone water is only ~5%.

And another more critical refinement is to account for for any (non circulating, and thereby cold) larger supply and return manifold piping volumes on your supply and return in gallons from the calculated loop gallons. This would not apply for a circulating and thereby hot primary loop.

If you calculate for example (by the above temperature difference method) that you have 4.27 total gallons of cold water in the loop, but on the loop you have 8 total feet of combined cold 1-1/4" supply and return manifold sections, then the combined 1-1/4" sections hold roughly 0.6 gallons, and your 3/4" section has only 4.27 - 0.6 = 3.67 gallons. This would yield an actual length of 137 ft. for only the 3/4" zone loop section.

3.67/0.0268 = 137 feet

If you can't see the pipe, how will you know what size it is?

Rob R. wrote:If you can't see the pipe, how will you know what size it is?

You can see it at the boiler at least. I've never seen hidden manifolds and circulators (but then again, to me all invisible men look alike).

If your results while assuming 3/4" pipe indicate a loop length of 40 feet where 150 feet is far more realistic, you probably have 1/2" pipe, etc... The only real curve ball would be for mixed diameter pipes within the walls or finished ceilings.

My result: 2 girth units